Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization

Diverse structural reactivity patterns of a POCOP ligand with coinage metals

We have utilised the 4,6-di-tert-butyl resorcinol bis(diphenylphosphinite) (POCOP) ligand for exploring its coordination ability towards group 11 metal centres. The treatment of the bidentate ligand 1 with various coinage metal precursors afforded a wide range of structurally diverse complexes 2-12, depending upon the metal precursors used. This furnishes several multinuclear Cu(I) complexes with dimeric (2) and tetrameric cores (3, 4, and 5). The tetrameric stairstep complex 4 shows thermochromic behaviour, whereas the dimeric complex 2 and tetrameric complex 3 show luminescence properties at cryogenic temperatures. Interestingly, the halide substitution reaction of the dimeric complex 2 with KPPh2 produces a unique mixed phosphine-based tetrameric Cu(I) complex, 5. Treatment of the POCOP ligand with [CuBF4(CH3CN)4] in the presence of 2,2'-bipyridine afforded heteroleptic complex 6, consisting of tri- and tetra-coordinated cationic Cu(I) centres. Furthermore, we could also isolate cubane (8) and stairstep (9) complexes of Ag(I). The cationic Au(I) complex (12) was obtained from the dinuclear Au(I) complex of POCOP, 11. Complex 12 revealed the presence of a strong intramolecular aurophilic interaction with an Au⋯Au bond distance of 3.1143(9) Å. Subsequently, the photophysical properties of these complexes have been studied. All the complexes were characterised by single-crystal X-ray diffraction studies, routine NMR techniques, and mass spectroscopy.

Design of luminescent complexes with different Cu4I4 cores based on pyridyl phenoxarsines

A series of luminescent Cu4I4 clusters with stair-step, cubane, and octahedral geometries supported by a novel type of cyclic As,N-ligand, pyridyl-containing 10-phenoxarsines, were synthesized and characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis. An unusual arrangement of As,N-bidentate and μ2-iodo ligands was found in the octahedral cluster. The structural diversity of the Cu(I) complexes is reflected in their photophysical properties: the phosphorescence spectra of the compounds display emission in a broad spectral range of 495-597 nm. The complex with the Cu4I4L2 stoichiometry bearing a stair-step Cu4I4 core demonstrates temperature-dependent dual emission. The luminescence properties of all complexes were rationalized by DFT calculations.

Photoluminescent copper(I) iodide alkylpyridine thin films as sensors for volatile halogenated compounds

The paper presents the fabrication and characterization of [CuI(L)]n thin films, where L represents various alkylpyridine ligands including 4-methylpyridine, 3-methylpyridine, 2-methylpyridine, 4-tbutylpyridine, 3,4-dimethylpyridine, and 3,5-dimethylpyridine. The thin films were synthesized by exposing the corresponding ligands to CuI thin films through vapor deposition. The coordination reactions occurring on the films were investigated using PXRD and time-dependent photoluminescence spectroscopy, and a comparison was made between the structures of the thin films and the corresponding powder phases. The films showed primarly blue emission (λem = 457-515 nm) and polymeric structures with excited state lifetimes ranging from 0.6 to 5.5 μs. Significantly, the studied compounds exhibited fast reversible luminescence quenching when exposed to vapors of dichloromethane and dibromomethane (15 and 30 min respectively), and the luminescence was restored upon re-exposure to the alkylpyridine ligand (after 20 min). These findings indicate that these thin films hold promise for applications as sensors (with sensitive and reversible detection capability) for volatile halogen-based compounds (VHC).

Luminescent Water-Dispersible Nanoparticles Engineered from Copper(I) Halide Cluster Core and P,N-Ligand with an Optimal Balance between Stability and ROS Generation

The present work introduces the solvent exchange procedure as a route for conversion of the Cu4I4L2 complex, where the Cu4I4 cluster core is coordinated with two P,N-ligands (L), into an aqueous colloid. The analysis of both colloidal and supernatant phases revealed some losses in CuI going from the initial Cu4I4L2 complex to Cu2I2L3-based nanoparticles. The comparative analysis of IR, 31P NMR spectroscopy, ESI mass-spectrometry and luminescence data argued for a contribution of the “butterfly”-like structures of the Cu2I2 cluster core to Cu2I2L3-based nanoparticles, although the amorphous nature of the latter restricted structure evaluation from the PXRD data. The green luminescence of the colloids revealed their chemical stability under pH variations in the solutions of some amino acids and peptides, and to specify the temperature and concentration conditions triggering the oxidative degradation of the nanoparticles. The spin trap-facilitated ESR study indicated that the oxidative transformations were followed by the generation of reactive oxygen species (ROS). The physiological temperature level (310 K) enhanced the ROS generation by nanoparticles, but the ROS level was suppressed in the solution of GSH at pH = 7.0. The cytotoxicity of nanoparticles was evaluated in the M-HeLa cell line and is discussed in correlation with their cell internalization and intracellular oxidative transformations.

Emission and Luminescent Vapochromism Control of Octahedral Cu4 I4 Complexes by Conformationally Restricted P,N Ligands

A conformationally restricted P,N-ligand capable of the design of polynuclear copper(I) complexes was synthesized via the reaction of primary pyridylphosphine, paraformaldehyde, and benzhydrylamine. The reaction of the ligand with copper(I) iodide leads to the tetranuclear copper(I) complex with the octahedral type of copper-iodide core. Different orientation of coordination bonds of the ligands relative to the P,N₂ -heterocyclic fragments and to the Cu₄ I₄ cores leads to the existence of two types of conformers of the complex with "compact" or "stretched" geometry of the Cu₄ I₄ cluster. This lability of the complex allowed for obtaining two crystalline phases displaying green or red luminescence. The TDDFT computations along with XRD structural analysis gave a strong interpretation of the green emission belonging to the "compact" form of the complex and belonging of the red emission to the "stretched" form. Moreover, both crystalline phases demonstrate the strong vapochromic responses of luminescence on the vapors of wide range of solvents.

Green Emissive Copper(I) Coordination Polymer Supported by the Diethylpyridylphosphine Ligand as a Luminescent Sensor for Overheating Processes

Tertiary diethylpyridylphosphine was synthesized by the reaction of pyridylphosphine with bromoethane in a suberbasic medium. The reaction of phosphine with the copper(I) iodide led to the formation of a copper(I) coordination polymer, which, according to the X-ray diffraction data, has an intermediate structure with a copper-halide core between the octahedral and stairstep geometries of the Cu₄I₄ clusters. The obtained coordination polymer exhibits a green emission in the solid state, which is caused by the ³(M+X)LCT transitions. The heating up of the copper(I) coordination polymer to 138.5 °C results in its monomerization and the formation of a new solid-state phase. The new phase exhibits a red emission, with the emission band maximum at 725 nm. According to the experimental data and quantum chemical computations, it was concluded that depolymerization probably leads to a complex that is formed with the octahedral structure of the copper-halide core. The resulting solid-state phase can be backward-converted to the polymer phase via recrystallization from the acetone or DMF. Therefore, the obtained coordination polymer can be considered a sensor or detector for the overheating of processes that should be maintained at temperatures below 138 °C (e.g., engines, boiling liquids, solar heat systems, etc.).

Copper(I)-Pyrazolate Complexes as Solid-State Phosphors: Deep-Blue Emission through a Remote Steric Effect

We describe a novel manifestation of rigidochromic behavior in a series of tetranuclear Cu(I)-pyrazolate (Cu₄pz₄) macrocycles, with implications for solid-state luminescence at deep-blue wavelengths (<460 nm). The Cu₄pz₄ emissions are remarkably sensitive to structural effects far from the luminescent core: when 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can induce a blue shift of more than 100 nm. X-ray crystal and computational analyses reveal that C4 units influence the conformational behavior of adjacent tert-butyl groups, with a subsequent impact on the global conformation of the Cu₄pz₄ complex. Emissions are mediated primarily through a cluster-centered triplet (³CC) state; compression of the Cu₄ cluster into a nearly close-packed geometry prevents the reorganization of its excited-state structure and preserves the ³CC energy at a high level. The remote steric effect may thus offer alternative strategies toward the design of phosphors with rigid excited-state geometries.

Recent Progress in Luminescent Cu(I) Halide Complexes: A Mini-Review

Copper(I) halide complexes are well sought-after materials due to their rich structural diversities and photophysical properties. Profoundly, there is a direct relationship between each structural variation and luminescence of these complexes, for a purported use. In this review, recent publications within the last 2 years about copper(I) halide complexes, centering on their structural dimensionalities with derivatives of nitrogen, phosphorus, and sulfur ligands, have been considered alongside their effects on luminescence.

Structural characterization and luminescence properties of trigonal Cu(i) iodine/bromine complexes comprising cation–π interactions

Trigonal copper(i) complexes comprising cation–π interactions achieve satisfactory photoluminescence properties.

Comprehensive Picture of the Excited State Dynamics of Cu(I)- and Ru(II)-Based Photosensitizers with Long-Lived Triplet States

Ru(II)- and Cu(I)-based photosensitizers featuring the recently developed biipo ligand (16H-benzo-[4',5']-isoquinolino-[2',1',:1,2]-imidazo-[4,5-f]-[1,10]-phenanthrolin-16-one) were comprehensively investigated by X-ray crystallography, electrochemistry, and especially several time-resolved spectroscopic methods covering all time scales from femto- to milliseconds. The analysis of the experimental results is supported by density functional theory (DFT) calculations. The biipo ligand consists of a coordinating 1,10-phenanthroline moiety fused with a 1,8-naphthalimide unit, which results in an extended π-system with an incorporated electron acceptor moiety. In a previous study, it was shown that this ligand enabled a Ru(II) complex that is an efficient singlet oxygen producer and of potential use for other light-driven applications due to its long emission lifetime. The goal of our here presented research is to provide a full spectroscopic picture of the processes that follow optical excitation. Interestingly, the Ru(II) and Cu(I) complexes differ in their characteristics even though the lowest electronically excited states involve in both cases the biipo ligand. The combined spectroscopic results indicate that an emissive ³MLCT state and a rather dark ³LC state are populated, each to some extent. For the Cu(I) complex, most of the excited population ends up in the ³LC state with an extraordinary lifetime of 439 μs in the solid state at 20 K, while a significant population of the ³MLCT state causes luminescence for the Ru(II) complex. Hence, there is a balance between these two states, which can be tuned by altering the metal center or even by thermal energy, as suggested by the temperature-dependent experiments.

Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes

The synthesis of copper and zinc complexes of four variably substituted iminophosphonamide ligands is presented. While the copper complexes form ligand-bridged dimers, the zinc compounds are monomeric. Due to different steric demand of the ligand the arrangement of the ligands within the dimeric complexes varies. Similar to the structurally related iminophosphonamide complexes of alkali metals and calcium, the steady-state and time-resolved photoluminescence (PL) of four of the seven compounds studied here as solids in a temperature range of 5-295 K can be described within the scheme of thermally activated delayed fluorescence (TADF). Accordingly, they exhibit bright blue-green phosphorescence at low temperatures (<100 K), which turns into delayed fluorescence by increasing the temperature. However, unusually, the fluorescence is practically absent in two copper complexes which otherwise still conform to the TADF scheme. In these cases, the excited singlet states decay essentially non-radiatively and their thermal population from the corresponding low-lying triplet states efficiently quenches PL (phosphorescence). Three other copper and zinc complexes only exhibit prompt fluorescence, evidencing a wide variation of photophysical properties in this class of compounds. The excited states of the copper complex with especially pronounced phosphorescence quenching were also investigated by low-temperature time-resolved infrared spectroscopy and quantum chemical calculations.

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2 L', Cu2 L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2 L' and Cu2 L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.

Investigation of Luminescent Triplet States in Tetranuclear CuI Complexes: Thermochromism and Structural Characterization

To develop new and flexible CuI containing luminescent substances, we extend our previous investigations on two metal-centered species to four metal-centered complexes. These complexes could be a basis for designing new organic light-emitting diode (OLED) relevant species. Both the synthesis and in-depth spectroscopic analysis, combined with high-level theoretical calculations are presented on a series of tetranuclear CuI complexes with a halide containing Cu4 X4 core (X=iodide, bromide or chloride) and two 2-(diphenylphosphino)pyridine bridging ligands with a methyl group in para (4-Me) or ortho (6-Me) position of the pyridine ring. The structure of the electronic ground state is characterized by X-ray diffraction, NMR, and IR spectroscopy with the support of theoretical calculations. In contrast to the para system, the complexes with ortho-substituted bridging ligands show a remarkable and reversible temperature-dependent dual phosphorescence. Here, we combine for the first time the luminescence thermochromism with time-resolved FTIR spectroscopy. Thus, we receive experimental data on the structures of the two triplet states involved in the luminescence thermochromism. The transient IR spectra of the underlying triplet metal/halide-to-ligand charge transfer (3 M/XLCT) and cluster-centered (3 CC) states were obtained and interpreted by comparison with calculated vibrational spectra. The systematic and significant dependence of the bridging halides was analyzed.

Keep it tight: a crucial role of bridging phosphine ligands in the design and optical properties of multinuclear coinage metal complexes

Copper subgroup metal ions in the +1 oxidation state are classical candidates for aggregation via non-covalent metal-metal interactions, which are supported by a number of bridging ligands. The bridging phosphines, soft donors with a relatively labile coordination to coinage metals, serve as convenient and essential components of the ligand environment that allow for efficient self-assembly of discrete polynuclear aggregates. Simultaneously, accessible and rich modification of the organic spacer of such P-donors has been used to generate many fascinating structures with attractive photoluminescent behavior. In this work we consider the development of di- and polynuclear complexes of M(i) (M = Cu, Ag, Au) and their photophysical properties, focusing on the effect of phosphine bridging ligands, their flexibility and denticity.

Transient FTIR spectroscopy after one- and two-colour excitation on a highly luminescent chromium(III) complex

The development of photoactive transition metal complexes with Earth-abundant metals is a rapidly growing research field, where a deeper understanding of the underlying photophysical processes is of great importance. A multitude of potential applications in the fields of photosensitizing, optical sensing, photoluminescence and photoredox catalysis motivates demanding spectroscopic studies. We applied a series of high-level spectroscopic methods on the previously reported highly luminescent chromium(iii) complex [Cr(ddpd)2](BF4)3 (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) possessing two near-IR emissive doublet states with microsecond lifetimes. Luminescence measurements were performed at temperatures down to about 10 K, showing a remarkable rise of the integrated emission intensity by more than a factor of three. The emissive doublet states were structurally characterized by transient FTIR spectroscopy at 290 K and 20 K, supplemented by ground state FTIR and Raman spectroscopy in combination with density functional theory. According to emission and step-scan FT-IR spectroscopy, the stronger luminescence at lower temperature arises from decreased non-radiative decay via energy transfer to CH vibrational overtones and increased radiative decay based on lowered symmetry. Pump/pump/probe (FTIR) and pump/dump/probe (FTIR) schemes were developed to modulate the excited doublet state populations at 290 and 20 K as a function of specific near-IR pump vs. dump wavelengths. The effect of the second near-IR pulse can be explained by combinations of excited state absorption, ground state absorption and stimulated emission. The successful establishment of these two-colour step-scan FTIR experiments is an important step towards profound studies on further transition metal complexes with energetically close-lying excited states in the near future.